Here's a pic of the last one I made to use for some ideas. I like the tines to be at least 9" long with about 1" 1 1/2" between tines.
I like 5/16" for the tines, adds a little more strength then 1/4", yet will push into a pike a little easier then 3/8". Put big barbs on it to start as it is easier to trim them down or heat them and bend them in, then it is to add on to them later. If a spear has an odd number of tines, double barb the center tine. If you decide to plasma cut a spear, round the sides of the tines real well otherwise after you spear a fish and it starts flopping around, the square corners will tear the hole even bigger and it will be easier for the pike to shake off.
Here's one sold by
http://www.darkhousespearingsupply.com/ that wouild be a good one for a pattern or give you some ideas.
If you build one like this, be careful not to sharpen the shoulder between the tines too much. I did that once and cut a pike in half when I speared it. Had to take a file and take the edge off of it.
Hope this helps, best of luck on your project.